Thermoplastic polymer extrusion bending

ABSTRACT

Methods for bending preformed thermoplastic extrusions having at least one cavity comprise filling at least one of the extrusion cavities with polymer foam and curing the foam within the filled cavity. An extrusion is heated to allow plastic deformation and then smoothly bent over a mandrill to impart a desired curved shape. Following cooling to below the extrusion&#39;s plastic deformation temperature on the mandrill, smoothly curved extrusions are removed for cooling to room temperature. Cured polymer foam within an extrusion cavity resists distortion of the cavity during the bending process.

BACKGROUND OF INVENTION

[0001] Thermoplastic polymer extrusions having one or more cavities orhollows (hereinafter “cavities”) are widely used, particularly inconstruction industries. Examples include rigid thermoplastic pipe andplastic framing materials for windows. In the latter applications, theextrusions protect, replace, and/or fit closely with associated woodenor metal structures and/or lower maintenance or construction costs. Suchextrusions are typically produced in straight preformed sections thatcan be bent as required for particular applications. But bending maycause kinking, cracking and/or distortion of the extrusions(particularly the cavities) that degrades their external appearanceand/or their fit with other structures. Several methods of avoiding orminimizing such bending-related problems are known.

[0002] For example, methods and apparatus for bending rigidthermoplastic pipe are discussed in U.S. Pat. Nos. 5,407,613 and5,593,708 (Schulte), and U.S. Pat. No. 5,765,285 (Buy, et al.), eachpatent incorporated herein by reference. Some of these methods includeheating the pipe to a temperature at which it can be plasticallydeformed, having previously inserted a flexible “snake” or otherinternal support (such as a coil spring) to prevent cross-sectionalnarrowing of the pipe cavity or pipe opening during bending. Subsequentcooling of the extrusion and removal of the supporting insert results inretention by the extrusion of the desired curved shape. A similartechnique has been practiced in bending vinyl extrusions for use inframing “arch top” or “round top” windows.

[0003] But these methods of extrusion bending have inherentdeficiencies. For example, there may be lot-to-lot variations in theinside dimensions of the extrusion cavities that make it difficult toinsert closely fitting snakes or other supports. Additionally, if asupport insert is removed while an extrusion is still at a temperaturewhere plastic deformation can occur, distortion of the extrusion mayresult. On the other hand, if a support insert is left in place too longduring the cooling phase after bending, it may be difficult orimpossible to remove without damage to the insert and/or the extrusion.

[0004] In the '285 patent referenced above, Buy, et al. propose to avoidproblems in bending pipe by eliminating the use of removable insertsaltogether. Instead, the '285 patent describes a close fitting metalsleeve or tube that is slipped over a pipe to form a composite structurethat can then be bent in a commercial tubing bender without kinking orcracking at the bend. Unfortunately, this method is poorly suited foruse with extrusions having relatively complex external shapes andcavities such as those commonly needed in window framing. Thus, a bettermethod of bending these more complex extrusions is needed.

SUMMARY OF INVENTION

[0005] Generally longitudinal cavities may be formed in an extrusion tosave on materials, to reduce weight and/or to mate with otherstructures. Depending on their shape and position in an extrusion,certain cavities may tend to become distorted if the extrusion is bent.The present invention relates to bending a preformed thermoplasticpolymer extrusion that comprises at least one cavity without materiallydistorting the extrusion cavity. By maintaining the size and shape ofcavities substantially constant during the bending process, methods ofthe present invention preserve important external appearances and/orfunctions of an extrusion in its new curved shape.

[0006] As noted in the Background, employing removable support insertssuch as springs or snakes to preserve the shape of selected extrusioncavities during bending is problematic. Methods of the present inventionavoid such problems by instead filling one or more cavities with polymerfoam that is easily formed within each such cavity. A preferredembodiment of these methods is schematically illustrated in theaccompanying drawing and discussed in greater detail below.

[0007] As foam is formed in an extrusion cavity from a mixture ofingredients, the foam expands to fill the available space, taking on thecavity's exact shape. Subsequent curing of the foam in the cavitycreates a new and closely-conforming internal support structure for thecavity. With an appropriate choice of compressive strength and/orrelated parameters such as density, the cured foam can be made to resistdistortion of the cavity during bending.

[0008] After curing of the foam, which is an exothermic process, theextrusion is heated as necessary to attain a first temperature(sometimes called the heat deflection or plastic deformationtemperature). After reaching this temperature, the extrusion can beplastically deformed as it is bent on a curved mandrill without kinkingor cracking. A smoothly curved bend is ensured because, notwithstandingthe heating and bending of the extrusion, the cured foam resistscross-sectional narrowing or distortion of cavities in which it has beencured.

[0009] The smoothly curved bend is further preserved by leaving thecurved extrusion on the mandrill while the extrusion is cooled to asecond temperature that is below its plastic deformation temperature.The cooling sets the smoothly curved extrusion's new shape, which willnot materially change if the extrusion is then removed from the mandrillfor further cooling to room temperature.

BRIEF DESCRIPTION OF DRAWING

[0010] The drawing graphically illustrates steps in a preferred methodof bending a representative thermoplastic vinyl polymer extrusion.

DETAILED DESCRIPTION

[0011] Referring to Step 10 on the drawing, the choice of ingredientsfor the foam-forming mixture is seen to be an early requirement inmethods of the present invention. In preferred embodiments, apolyisocyanate based foam may be formed by pouring hand-mixedingredients in each desired cavity. Alternatively, foam may be formed byinjecting directly into the cavity (as from a mixing head) a mixturecomprising an organic polyisocyanate, one or more activehydrogen-containing compounds, and one or more expansion agents (blowingagents). Descriptions of polymer foam formation and curing, includingexamples of the above ingredients and additional components of themixture such as catalysts and surfactants, are provided in U.S. Pat.Nos. 4,943,597 and 4,972,003 (Grunbauer, et al.) and U.S. Pat. No.5,164,419 (Bartlett, et al.), each patent incorporated herein byreference.

[0012] Whatever the mixture ingredients chosen may be, preparation of afoam that exhibits attractive physical properties is easier if theingredients are readily miscible with one another. Further,high-efficiency mixing procedures are preferably employed to ensure aneven distribution of all starting materials.

[0013] Since the polymerization reaction that initiates foam formationis exothermic, heat is available to vaporize small encapsulated pocketsof one or more liquid blowing agents that may be present. These smallamounts of vapor expand to form bubbles in the liquid phase of thepolymerization reaction, and the bubbles become foam cells aspolymerization progresses. Thus, a polymer foam is created that, inpreferred embodiments, subsequently cures as polymerization is completedwithin the cavity (Step 12) to form a polyurethane foam.

[0014] Cured polymer foam may have a range of physical properties (e.g.,density, cell size, compressive strength, friability, and flexibility),depending largely on the specific choices of ingredients for the abovemixture. Because of the wide range of demands that may be placed on thefoam in preventing distortion of extrusion cavities during bending, aswell as the different conditions that may exist during foam curing, nosingle foam formulation will necessarily be satisfactory for allapplications.

[0015] For example, the amount of heat produced during thepolymerization reaction is important for at least two reasons. First,some of the heat may be used to vaporize liquid blowing agents. Second,depending on the size and number of extrusion cavities to befoam-filled, enough heat may be generated as the foam is formed andcured to raise portions of the polymer extrusion's temperaturesignificantly above, for example, about 70 degrees Celsius. Forpreferred vinyl polymer extrusions, 70 degrees Celsius is near the heatdeflection (or plastic deformation) temperature.

[0016] Thus, 70 degrees Celsius is indicated as representative of thefirst temperature T₁ in Step 14. It is near the actual preferred bendingtemperature for a representative class of vinyl extrusions to whichmethods of the present invention may be applied.

[0017] The temperature at which these same vinyl compositions wereoriginally extruded (about 160 degrees Celsius) is only about 90 degreesCelsius above the heat deflection temperature. As a vinyl extrusionapproaches its own extrusion temperature, the risk of catastrophicdistortion increases significantly. Effective control of overallextrusion temperature rise during foam filling and curing is a functionof extrusion geometry as well as the chosen polymerization reaction.Important effects of this temperature rise on shaping a polymericextrusion are disclosed in U.S. Pat. No. 6,319,456 (Gilbert, et al.),incorporated herein by reference.

[0018] Another factor affecting extrusion temperature prior to bending,of course, is any external heat that may be applied. For example,relatively close temperature control may be maintained by a “wet”process, e.g., immersing the extrusion in a temperature-controlledglycol bath. Temperature control may also be achieved if an extrusion isheated by a “dry” process, e.g., placing it in a stream of heated air orunder an infrared (radiant) heater. These processes allow achievement ofsubstantial temperature equilibrium throughout an extrusion to obtainpredictable and controlled plastic deformation during bending.

[0019] However, in some cases it may be desirable to allow temperaturegradients to exist in an extrusion during bending on a curved mandrill(Step 18). If portions of an extrusion that experience the greateststress during bending are also in a relatively more plastic state, theseportions may beneficially experience the greatest stress relief.Simultaneously, other (less plastic) portions can serve to stabilize theoverall structure during bending. See the '456 patent referenced abovefor an example of this technique.

[0020] The stress relief resulting from selective stretching of portionsof a polymer extrusion during bending may also improve the localmolecular structure of the polymer, conferring additional strength andhardness under certain conditions. See U.S. Pat. No. 5,597,185 (Bray, etal.), for example. In this manner, the additional localized heatingaround cavities generated by the polymerization reaction may be used toadvantage in certain embodiments.

[0021] Temperatures throughout an extrusion, including any foam-filledcavities, must be monitored or accurately estimated (Step 22) duringcooling (Step 20) to ensure there will be no undesired distortion afterthe extrusion is removed from the mandrill. Yet to shorten productiondelays, extrusions will preferably be removed from the mandrill (Step24) soon after critical portions are safely below the heat deflectiontemperature (i.e., at a temperature T₂ below about 60 degrees Celsiusfor preferred formulations of vinyl extrusions).

[0022] Providing relatively greater stress relief to the more highlystressed portions of an extrusion undergoing bending may reduce therequirements for cavity stabilization that are placed on the foam. Foamusable in methods of the present invention preferably ranges in densityfrom about 16 kg per cubic meter to about 320 kg per cubic meter. Foamdensity is an important determinant of the amount of stabilization thefoam can provide to extrusion cavities, but foam cell size, the ratio ofopen to closed cells, and the resilience (or friability) of the foam mayalso be important. All of these parameters can be influenced by thechoice of ingredients in the foam-forming mixture.

[0023] Example foam preparations are described in U.S. Pat. No.4,218,543 (Weber, et al.), incorporated herein by reference.

1. A method for bending a preformed thermoplastic polymer extrusioncomprising at least one cavity to make a curved polymer extrusion, themethod comprising: filling at least one said cavity with polymer foamformed within said cavity; curing said polymer foam within said at leastone cavity; heating said extrusion to a first temperature; bending saidheated extrusion on a curved mandrill; cooling said extrusion to asecond temperature on said mandrill to make a curved polymer extrusion;and removing said cooled curved polymer extrusion from said mandrill. 2.The method of claim 1 wherein said polymer foam is polyisocyanate-based.3. The method of claim 2 wherein said polymer foam is polyurethane foam.4. The method of claim 3 wherein said polyurethane foam is rigidclosed-cell foam, semi-rigid closed-cell/open-cell foam and flexibleopen-cell foam.
 5. The method of claim 1 wherein said first temperatureis the heat deflection temperature of the preformed polymer extrusion.6. The method of claim 1 wherein said second temperature is at leastabout 10 degrees Celsius less than the heat deflection temperature ofthe preformed polymer extrusion.
 7. The method of claim 1 wherein saidpolymer foam has a density of about 16 kg per cubic meter to about 320kg per cubic meter.
 8. The method of claim 1 wherein said extrusion isheated to said first temperature in a glycol bath.
 9. The method ofclaim 1 wherein said extrusion is heated to said first temperature byinfrared radiation.
 10. The method of claim 1 wherein said extrusion isheated to said first temperature by heated air.
 11. The method of claim1 wherein said preformed extrusion comprises a vinyl polymer.
 12. Themethod of claim 1 wherein each said cavity is filled with foam byinjection from a mixing head of a plurality of ingredients comprisingpolyisocyanate, at least one active hydrogen-containing compound, and ablowing agent.
 13. The method of claim 1 wherein each said cavity isfilled with foam by hand pouring into each said cavity a plurality ofingredients comprising polyisocyanate, at least one activehydrogen-containing compound, and a blowing agent.
 14. A method forbending a preformed vinyl extrusion comprising at least one cavity tomake a curved vinyl extrusion, the method comprising: filling at leastone said cavity with polyurethane foam formed within said cavity; curingsaid polyurethane foam within said at least one cavity; heating saidextrusion to about 70 degrees Celsius; bending said heated extrusion ona curved mandrill; cooling said extrusion to a temperature less thanabout 60 degrees Celsius on said mandrill to make a curved polymerextrusion; and removing said cooled curved polymer extrusion from saidmandrill.
 15. The method of claim 14 wherein said extrusion is heated byimmersion in a glycol bath maintained at about 70 degrees Celsius. 16.The method of claim 14 wherein said extrusion is heated by infraredradiation.
 17. The method of claim 16 wherein said cured polyurethanefoam has a density of about 320 kg per cubic meter.
 18. A window framecomprising a curved polymer extrusion made by the method of claim 14.19. A window comprising the window frame of claim
 18. 20. The window ofclaim 19 wherein said polyurethane foam is rigid closed-cell foam,semi-rigid closed-cell/open-cell foam and flexible open-cell foam.